Alignment films in a liquid crystal display device and a method of manufacturing the same

ABSTRACT

A liquid crystal display device including a pair of substrates in a spaced relationship with one another. A pair of alignment films are provided, one alignment film being formed on each substrate such that the alignment films face one another. A liquid crystal layer; including plural liquid crystals, is inserted between the pair of alignment films, wherein the alignment films impart a given pre-tilt angle to the liquid crystals. The alignment films are composed of a material containing at least two types of polymers having a prescribed initial alignment and different alignment variation rates in response to ultraviolet ray irradiation. The pre-tilt angle being adjusted, without rubbing the alignment films, through ultraviolet exposure of the alignment films.

This is a divisional of application Ser. No. 10/422,253 filed Apr. 24,2003, which is a divisional of application Ser. No. 09/629,287 filedJul. 31, 2000, now U.S. Pat. No. 6,583,835.

FIELD OF THE INVENTION

The present invention relates to alignment films which orient liquidcrystals provided between the alignment films in a liquid crystaldisplay device, and a method for manufacturing the alignment films.

BACKGROUND OF THE INVENTION

In recent years, liquid crystal display devices, particularly, Thin FilmTransistor (TFT) liquid crystal display devices which have a twistednematic (TN) display mode, have come into wide use. For example, theyare the general-purpose display devices in personal computers.

Usually, a liquid crystal display device includes a pair of opposingsubstrates that are maintained with a prescribed interval, electrodesand alignment films formed on the facing surfaces of the substrates, anda liquid crystal layer inserted between the alignment films. Theelectrodes of one substrate are formed into a common electrode. Theelectrodes of the other substrate are formed into the pixel electrodes.The pixel electrodes are often provided with an active matrix. Inaddition, electrodes are provided only on one substrate (for example,IPS mode). A black matrix or color filter is provided on eithersubstrate.

In conventional liquid crystal display devices, the liquid crystalmolecules in the liquid crystal layer are oriented in the prescribeddirection by rubbing the alignment film. The alignment film is polishedby a cloth, for example rayon, which undesirably generates dust withinthe clean room. Moreover, the rubbing generates static electricity whichcould potentially result in the breakdown of the TFT of the activematrix.

The inventors of the present invention have proposed in Japanese patentapplication HEI 9-354940 and Japanese patent application HEI 11-72085 atechnique for orienting the liquid crystal molecules through the use ofultra-violet rays. As illustrated in FIG. 37, ultraviolet light isirradiated at an angle of 45°, for example, with respect to the surfaceof the polyimide alignment film 501, thereby orienting the liquidcrystal molecules 502.

The relationship between the pre-tilt angle and the amount ofultra-violet ray irradiation realized by the method of Japanese patentapplication 11-72085 is illustrated in FIG. 38.

From the relationship shown in the drawing, when the volume ofultraviolet ray irradiation is low and the pre-tilt angle is large,black points occur in locations centered around spacers used maintainthe spacing between the substrates (cell gap) of the liquid crystaldisplay device. Correspondingly, if the ultraviolet light exposure ishigh, flow-induced orientations accompanying the injection of liquidcrystal are produced. Both arc primary causes of poor displays. In thiscase, an appropriate range for the pretilt angles that obtain displayshaving good images is a narrow range of no more than 1.0° centered near89°

One of the problems associated with the teachings of Japanese patentapplication HEI 11-72085 is strong reliance on the proper control of theangle and intensity of the ultra-violet rays. Optimum results require amaximum of a ±10% intensity deviation in order to obtain a givenpre-tilt angle. Referring to the properties curve of FIG. 8, a deviationof ±0.2% commonly occurs both in the angle of irradiation and in theintensity of the ultra-violet rays, making it difficult to reliablyobtain a specified pre-tilt angle. Consequently, the probability of poordisplay occurring increases and there is a concern that the display willbe unreliable.

Accordingly, an object of the present invention is to provide animproved method for orientation of an orientation film in which adesired pre-tilt angle of liquid crystal molecules can be assuredwithout the need for rubbing the orientation film, and which does notsuffer from the aforementioned problem relating to proper control of theangle and intensity of the ultra-violet rays.

Another object of the present invention is to increase the contrast inthe display surface and prevent light and dark reversal in the display,and provide an alignment technique that exposes the alignment film withultraviolet light from different directions and produces domains in thepixels. As shown in FIGS. 39A and 39B, two domains are created in thealignment film 611 by using an optical mask 601 formed with a slit 602.The optical mask 601 is placed above the alignment film 611, andparallel ultraviolet light is irradiated at an incline from above theoptical mask 601. Next, parallel light is irradiated again at an inclinehaving a different angle (Unexamined Japanese Patent Publication (Kokai)No. Hei 11-133429). Thus, the alignment film 601 is irradiated multipletimes, one time for each domain. Naturally, this leads to an increase inthe number of processes.

The method disclosed in Hei 11-133429 is further problematic as themultiple irradiations tend to cause bending of the optical mask. Asshown in FIG. 40, bending of the optical mask 601 causes offsets in theexposure positions of the ultraviolet light on the alignment film 611.For example, even if the ultraviolet light is irradiated in twodirections with the center of the pixel as the boundary, the domain willhave an offset center position. The size of the glass substrate hastended to increase each year, and the use of a 1 m² substrate isexpected. If the thickness of the optical mask is 1 cm, bending byseveral dozen at the center of the optical mask will be apparent basedon calculations. Thus, design offsets that cannot be ignored will occur.

Consequently, if domains are created as described above, in addition tothe difficulty in controlling the angle and intensity of the irradiatedultraviolet light, the processes will necessarily become more complex.

In view of the problems described above, another object of the presentinvention is to provide a liquid crystal display device that has asimple structure and is provided with alignment films that can verystably and easily obtain the appropriate pretilt angles for the liquidcrystal molecules by a simple alignment process without rubbing.

Another object of the present invention is to provide an alignmentapparatus, an alignment method able to easily and accurately createdomains without increasing the number of processes.

The present invention provides an improved liquid display device and amethod for creating the same. According to a first embodiment, theliquid crystal display device includes a pair of substrates in a spacedrelationship with one another. A pair of alignment films are provided,one alignment film being formed on each substrate such that thealignment films face one another. A liquid crystal layer, includingplural liquid crystals, is inserted between the pair of alignment films,wherein the alignment films impart a given pre-tilt angle to the liquidcrystals. The alignment films are composed of a material containing atleast two types of polymers having a prescribed initial alignment anddifferent alignment variation rates in response to ultra-violet rayirradiation. The pre-tilt angle being adjusted, without rubbing thealignment films, through ultraviolet exposure of the alignment films.

Also disclosed is an alignment apparatus for adjusting the alignment ofan alignment film with ultraviolet light. The alignment apparatusincludes a light source to irradiate scattered ultraviolet light, and anoptical mask disposed under the light source. The optical mask is formedwith at least one slit. In operation the optical mask is placed abovethe alignment film and scattered ultraviolet light irradiates from thelight source through the optical mask. Diffuse light exposes thealignment film, and produces domains in the liquid crystal that dependon the directions of diffusion of the diffuse light.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-described objects of the invention will be apparent from thefollowing detailed description of the invention, while referring to theattached drawings in which:

FIG. 1 is a cross-sectional view showing the overall structure of aliquid crystal display device according to one embodiment of the presentinvention;

FIG. 2 is a characteristic plot showing changes in pre-tilt anglesstates for alignment films with respect to the ultraviolet lightexposure;

FIG. 3 is a characteristic plot showing the surface free energy changeswith respect to the ultraviolet light exposure for alignment filmscomposed of one polymer;

FIG. 4A is a characteristic plot showing the surface free energy changeswith respect to the ultraviolet light exposure for alignment filmscomposed of one polymer;

FIG. 4B is a characteristic plot showing the surface free energy changeswith respect to the ultraviolet light exposure for several differentalignment films;

FIG. 4C is a characteristic plot showing changes in pre-tilt anglesstates for several different alignment films with respect to theultraviolet light exposure;

FIG. 5 is a schematic diagram of an apparatus for irradiating analignment film according to the present invention;

FIG. 6 is a characteristic plot showing the ideal changes in the pretiltangle with respect to the ultraviolet light exposure for the alignmentfilms in a liquid crystal display device according to the presentinvention;

FIG. 7 is a cross-sectional view showing the main structures of anapparatus for irradiating an alignment film according to anotherembodiment;

FIG. 8 is a cross-sectional view showing the state of the ultravioletlight exposure when the optical mask is bent;

FIG. 9 is a characteristic plot shows the changes in the pretilt angleaccompanying the ultraviolet light irradiating the alignment film of thepresent invention;

FIGS. 10A and 10B are cross-sectional views shows the set up of therib-shaped parts in a substrate when there are two domains;

FIG. 11 is a schematic diagram showing the structure of the light sourceused in FIG. 7;

FIG. 12 is a diagram showing the light source of FIG. 11 scanning anoptical mask;

FIGS. 13A-13C are schematic diagrams of a TFT LCD having top and bottomdomains;

FIGS. 14A-14C are schematic diagrams of a TFT LCD having left and rightdomains;

FIGS. 15A-15C are schematic diagrams of a TFT LCD having top, bottom,left, and right domains;

FIGS. 16A-16C are schematic diagrams of a TFT LCD having top, bottom,left, and right domains;

FIGS. 17A-17C are schematic diagrams of a TFT LCD having top and bottomdomains;

FIG. 18 is a cross-sectional view showing the relationship of thepositions of the rib-shaped part and the pixel electrode;

FIG. 19 is a characteristic plot showing the relationship between thewidth of the overlap of the rib-shaped part and the pixel electrode andthe width of the poor alignment it the end of the pixel electrode;

FIGS. 20A-C are schematic diagrams showing a modification of the TFT LCDof FIG. 13;

FIG. 21 is a characteristic plot showing optimum values for the width ofthe slit in the optical mask with a favorable alignment state and thedistance between the optical mask and the substrate;

FIG. 22 is a cross-sectional view shows the important structures of thealignment apparatus according to a third embodiment;

FIG. 23 is a cross-sectional view showing a first modification of thestructure shown in FIG. 22;

FIG. 24 is a cross-sectional view showing a second modification of thestructure shown in FIG. 22;

FIGS. 25A and 25B are cross-sectional views shows the importantstructures of an alignment apparatus according to a fourth embodiment;

FIG. 26 is an oblique projection view showing the optical mask in thealignment apparatus;

FIG. 27 is an oblique projection view showing the placement state of theoptical mask;

FIGS. 28A and 28B are top views showing the image state in the liquidcrystal display device according to the fourth embodiment;

FIGS. 29A and 29B are top views showing the image state of the liquidcrystal display device using only alignment control for two divisions;

FIG. 30 is an oblique projection view showing the optical mask in thealignment apparatus according to a first modification;

FIG. 31 is an oblique projection view showing the optical mask in thealignment apparatus according to a second modification;

FIGS. 32A and 32B are schematic diagrams of the light source used in thealignment apparatus of FIGS. 25A and 25B;

FIG. 33 is a top view shows the relationship between the scatteringcharacteristics of the light source and the slit in the optical mask;

FIG. 34 is a top view shows the vicinity of a pixel electrode in theliquid crystal display device in a fifth embodiment;

FIGS. 35A and 35B are cross-sectional views showing orientations of theliquid crystal molecules in the pixel electrode formed with a slit;

FIGS. 36A and 36B are schematic diagrams of another example of a fifthembodiment;

FIG. 37 is a schematic showing the pretilt angle orientation of liquidcrystal molecules by alignment films exposed to oblique ultravioletlight;

FIG. 38 is a characteristic plot showing changes in the pretilt anglewith respect to the ultraviolet light exposure for alignment filmscomposed of one polymer;

FIGS. 39A and 39B are cross-sectional views showing two domainsimplemented in a conventional alignment film; and

FIG. 40 is a cross-sectional view is used to describe the problems whenthe optical mask is bent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various specific embodiments applying the present invention areexplained in detail while referring to the drawings.

FIG. 1 is a cross-sectional view of a liquid crystal display deviceaccording to a first embodiment. The liquid crystal display deviceincludes a pair of opposing transparent glass substrates 11, 12 with aliquid crystal layer 13 interposed therebetween.

A plurality of pixel electrodes 15 are formed on an interveninginsulation layer 14 provided on transparent glass substrate 11, and atransparent alignment film 16 a covers the pixel electrodes 15. A colorfilter 17, a common electrode 18, and an alignment film 16 b aresuccessively layered on the transparent glass substrate 12. Thealignment films 16 a, 16 b push towards each other to hold the liquidcrystal layer 13, and the glass substrates 11, 12 are fixed.

Polarizers 19, 20 are provided on the outer sides of the substrates 11,12. The pixel electrodes 15 are formed with the active matrix. In theillustrated example, the data bus lines 21 in the active matrix areshown. The electrodes are provided on only one substrate (for example,in the IPS mode).

The alignment film 16 a (1 6 b) provides a prescribed orientationproperty for the liquid crystal molecules in the liquid crystal layer13. The prescribed orientation is realized by irradiating ultravioletlight from an inclined direction to the liquid crystal layer 13 withoutrubbing.

Specifically, the alignment film 16 a (16 b) is composed of a materialcontaining two polymers x1, x2 having different rates of change of thepretilt angles in response to ultraviolet light exposure. Polymer x1responds extremely rapidly to ultraviolet light, and its pretilt anglerapidly decreases under a small ultraviolet light exposure. In contrast,polymer x2 responds extremely slowly to ultraviolet light and thepretilt angle is hardly changed at all by ultraviolet light exposure.Using mixtures or copolymers of three or more polymers having differentrates of change of the pretilt angle may achieved.

FIG. 38 is a graph showing the relationship between the ultravioletlight exposure (J/cm²) and the pretilt angle (°) for a given polymer.Notably, the change in the pretilt angle is large with respect to slightchanges in the ultraviolet light exposure, making it difficult toachieve a desired pretilt angle. Ideally, the pretilt angle shouldrapidly decreases to a desired value under a small ultraviolet lightexposure, and thereafter maintain the desired pretilt angle irregardlessof additional ultraviolet light exposure.

As shown in FIG. 2, the alignment film 16 a (16 b) is formed by usingpolymer x1 which rapidly decreases the pretilt angle under ultravioletlight exposure (J/cm²) and polymer x2 which does not change the pretiltangle and has almost no dependence on the ultraviolet light exposure.

The polymer for alignment film 16 a (16 b) is a vertically alignedpolyimide or polyamic acid. An example is shown below.

The polymer considered has the alkyl side chain (alkyl group) R as shownin formula 1 and randomly projects to the surface of the alignment film16 a (16 b). If ultraviolet light irradiates the surface,photodecomposition develops and breaks the straight chain that supportsthe alkyl side chain R which essentially reduces the alkyl side chain Rand subsequently appears as a decrease in the pretilt angle of theliquid crystal molecules. Polymer xl has a structure in which thestraight that supports the alkyl side chain R is remarkably easy tobreak compared to polymer x2. Specifically, a region wherephotodecomposition easily occurs, for example, a double bond region, isprovided as the straight chain supporting the alkyl side chain R of thepolymer x1. If ultraviolet light irradiates this double bond region,photo-decomposition develops even for an extremely small exposure andcauses a substantial decrease in the pretilt angle in a short time.

Assuming, for the purposes of illustration, a copolymer composed oftwenty percent polymer x1 and eighty percent polymer x2. When theultraviolet light exposure is initiated, the state of polymer x1 withinthe copolymer rapidly changes until the amount of the alkyl side chain Rthat manifests the pretilt angle is reduced to essentially 0. Incontrast, polymer x2 within the copolymer maintains its initial state ofvertical alignment because there is no double bond region in thestraight chain of the alkyl side chain R. Therefore, the overallcopolymer maintains a nearly constant pre-tilt value after the specifiedtime has elapsed in which the amount of the alkyl side chain R isdecreased to 80 percent.

The criteria for selecting polymers x1 and x2 to obtain a suitablealignment film having the characteristics described above will now bedescribed.

FIG. 3 shows the relationship between the ultraviolet light exposuretime (minutes) on the alignment film surface and the surface free energy(γs: Helmholtz free energy per unit area). If a small amount ofultraviolet light is irradiated, the surface energy also becomes smalland the surface free energy increases along with the ultraviolet lightirradiation and finally becomes a nearly constant value.

As shown in FIG. 4A, if the surface free energy increases with theincrease in the ultraviolet light exposure time, the so-called injectionstripe from the injection port is generated when the liquid crystal isinjected. Furthermore, when the exposure time increases, the verticalalignment is not exhibited at the earliest time and the alignment movesto a random horizontal alignment.

The present inventors discovered that the surface free energyaccompanying the increase in the ultraviolet light exposure time (amountof exposure) successively moves from region (1) to region (4). Region(1) is the initial state exhibiting vertical alignment, region (2)exhibits good image display with no generation of flow-inducedorientation defects or defects near the spacers, region (3) exhibitsinjection strips caused by flow-induced orientations, and region (4)exhibits horizontal alignment.

As shown in FIG. 4B, the alignment film may be classified according tosurface free energy into three general types. Alignment film A rapidlymoves into region (4) in response to a small ultraviolet light exposure(short time) and has random horizontal alignments. Alignment film Bmoves into region (3) after a predefined amount of ultraviolet lightexposure, and at a generally slower rate than alignment film A.Alignment film C generally remains in region (1) regardless of theamount of ultraviolet light exposure. Moreover, the desired state(Region (2)) is implemented by properly combining alignment films A toC.

FIG. 4C shows the relationship between the ultraviolet light exposureand the pretilt angle for the combinations of alignment films A and Band alignment films A and C. If alignment films A and B are combined,the pretilt angle continues to slowly decrease as the ultraviolet lightexposure increases and does not contribute to an increase in the margin,and acceptable orientations are not achieved. In contrast, if alignmentfilms A and C are combined, a region in which the pretilt angle hardlychanges is created even if the ultraviolet light exposure changes, andorientations having a wide margin are achieved.

As described above, suitable polymers x1 and x2 exhibit both extremeproperties related to the manifestation of the pretilt angle. In otherwords, polymer xl will have random horizontal alignments caused by asmall ultraviolet light exposure (short time period). In contrast,polymer x2 still maintains the initial vertical alignment. Thus, withthe surface free energy as the criterion, alignment film A (polymer) isappropriately selected to be polymer x1 and alignment film C to bepolymer x1.

Next, the alignment method which is the main process of the embodimentin the method for manufacturing liquid crystal display devices isdescribed.

Referring back to FIG. 1, after the insulation film 14 is deposited in alayer on the surface of the transparent glass substrate 11, the colorfilter 17 and the pixel electrodes 15 are successively formed on thesurface of the transparent glass substrate 12.

Next, a vertically aligned polyimide or polyamic acid (see formula 1)manufactured by Japan Synthetic Rubber Ltd. is used for polymers x1 andx2 having the above properties on the surfaces of the transparent glasssubstrates 11, 12. According to a preferred embodiment, polymers x1 andx2 are mixed or copolymerized at a 2:8 ratio and form the alignmentfilms 16 a, 16 b on the surfaces of the transparent glass substrates 11,12. However, different ratios may be selected depending on the desiredpre-tilt angle.

FIG. 5 illustrates an alignment apparatus useful for implementingalignment processing on the target film.

The alignment apparatus includes of a light source 31 to irradiatenon-polarized ultraviolet light, a mirror 32, and a holder 33 forsupporting the transparent glass substrate 11 (12) forming the alignmentfilm 16 a (16 b). The holder 33 supports the transparent glass substrate11 (12) at an incline with respect to the optical axis of theultraviolet light. The parallel ultraviolet light from the light source31 is incident at an angle of θ=45° with respect to the surface of thealignment film 16 a (16 b) (or at a specified angle less than 45°).

The light source 31 is a short-arc xenon mercury lamp, includes aparabolic reflector 31 a, and exposes nearly parallel non-polarizedultraviolet light. The spectral distribution of the ultraviolet lightwavelengths has a peak near 250 nm. In this spectral distribution, thewavelength components at and above 300 nm are judged to not contributeto appearance of the pretilt angle. Ultraviolet light having awavelength no more than 280 nm is suited to effectively producing thepretilt angle. The P-waves and S-waves for the polarized ultravioletlight to be irradiated can have the state with more P-waves than S-wavesor the state with only P-waves.

The alignment apparatus having the above structure irradiatesultraviolet light from an angle of 45° at an incline with respect to thesurface of the alignment film 16 a (1 6 b). Polymer xl decreases thepretilt angle under several dozen mJ/cm² of ultraviolet light exposure.Polymer x2 produces no change in the pretilt angle even when exposed toseveral J/cm² of ultraviolet light. Therefore, the ultraviolet lightexposure is set to 1 J/cm².

The reliable demonstration of the above properties of polymers x1 and x2is considered here. A suitable relationship between the ultravioletlight exposure and the pretilt angle is at least a 2° change in thepretilt angle for ultraviolet light exposure no more than 0.5 J/cm² forpolymer x1, and a change of no more than 0.5° in the pre tilt angle forno more than 1 J/cm² ultraviolet light exposure for polymer x2.

As shown in FIG. 6, when the ultraviolet light was actually irradiatedunder these conditions, a stable pretilt angle around 89° could beobtained. Fluctuations in the pretilt angle in the ultraviolet lightexposure range of 1±0.3 J/cm² were no more than 0.1°. Consequently, evenif fluctuations arise in the amount of exposure of ultraviolet light,the desired pretilt angle is obtained.

Next, the liquid crystal is injected between the pair of transparentglass substrates 11, 12 to form the liquid crystal layer 13, then theinjection port is sealed. After hardening, various post processes, whichdo form part of the claimed invention, are performed to finish theliquid crystal display device.

As described above, an alignment film 16 a (16 b) imparting a desiredpre-tilt may be easily and reliably achieved (without rubbing).

A method and apparatus for producing domains in the alignment films willnow be explained with reference to FIG. 7.

The alignment apparatus includes a light source 101 that irradiatesscattered ultraviolet light and an optical mask 102 that is placed belowthe light source 101 and is formed with a slit 111.

The light source 101 is an ultraviolet lamp having the property ofscattering light. For example, a tubular low-pressure mercury lamp isone version. Its shape is similar to an ordinary long fluorescent lamp,but the gas component or glass material for the heavy glass tubediffers. The ultraviolet light particularly near the wavelength of 250nm is irradiated as scattered light.

The optical mask 102 is disposed at a constant distance from the coatingof the alignment film 103 or the printed substrate 104, for example,separated by approximately 50 μm. A slit 111 in the optical mask 102 isformed to transmit the scattered ultraviolet light. If the light source101 is a mercury lamp and scans in the direction indicated by the arrowin FIG. 7 above the optical mask 102, diffuse light that spreads outcentered on the slit 111 is produced. The diffuse light exposes thealignment film 103, and two domains that depend on the directions ofdiffusion of the incline of the diffuse light are created with theregion directly under the slit 111 as the boundary. Notably, two domainshaving different inclines are created by a single ultraviolet lightexposure.

Light source 101 symmetrically irradiates the diffuse ultraviolet lightfrom an inclined direction with respect to the surface of the alignmentfilm 103 about the center of symmetry which is the region directly belowthe slit 111 of the optical mask 102. Therefore, two domains arcautomatically created in the alignment film 103 with the center ofsymmetry as the boundary. In this case, the exposure angle changes asthe diffuse light moves away from the center of symmetry to obtain aliquid crystal layer with superior visual characteristics and aplurality of pretilt angles. The resulting alignment film has thefollowing properties: (1) the directions in which the liquid crystalmolecules fall in mutually opposite directions, (2) the alignment in thecentral region where the molecules fall is the vertical alignment, and(3) the magnitude of the surface energy of the alignment film becomeslarger or smaller closer to the slit. Therefore, a liquid crystaldisplay device provided with this alignment film produces multipledomains at the specified boundaries in the liquid crystal, and thesurface energy of the alignment film reaches a maximum or a minimum atthe boundaries of the domains and either decreases or increases whenmoving away from the boundaries.

The expected domains are produced without being significantly affectedusing the above-described structure even if the optical mask 102 bends.By manner of illustration, FIG. 8 shows that the center of symmetry ofthe irradiating light does not change even if the optical mask 102 isbent because the original scattered light falls incident perpendicularto the optical mask 102. However, because the region of the incominglight changes, this margin must be estimated to design the gap betweenthe optical mask 2 and the substrate and the width of the slit 111.

The alignment film 103 is preferably a copolymer of the type describedpreviously. Specifically, a copolymer of two or more polymers selectedto provide a pretilt angle having a constant value near 90° when theultraviolet light exposure exceeds some level. By using alignment filmshaving this property, the vertical alignment is maintained directlybelow the slit 111 and the pretilt angle of the liquid crystal layer hasa stable distribution from 90° to the constant value in response to theexposure angle and the amount of exposure of the scattered light. Theultraviolet light exposure is finished in one iteration.

In addition, the preferred alignment film has the characteristic ofstarting to change the pretilt angle from an initial vertical alignmentin response to ultraviolet light exposure, and returning to the verticalalignment by exposing ultraviolet light again.

FIG. 9 shows the changes in the pretilt angle accompanying theultraviolet light exposure of the alignment film. As the ultravioletlight exposure increases, the alignment moves from non-vertical to avertical alignment. As described above, the alignment rapidly changesfrom the initial (non-vertical) state to a final (vertical) state inresponse to UV exposure, and thereafter generally maintains the final(vertical) state regardless of additional UV irradiation. Notably,fluctuations in UV irradiation cause only minor changes in alignment.

In this case, because the alignment does not become the horizontalalignment even directly below the slit 111, where a lot of ultravioletlight is irradiated, the alignments do not become disordered. It shouldbe appreciated that the use of a conventional film would result inhorizontal alignment below slit 111 where a great deal of ultravioletlight is exposed, resulting in a region with poor alignment that emitswhite light results even in the black display state. In contrast, usingthe alignment film of the present invention, the alignments arecontinuous even in regions such as below slit 111, and poor alignmentsare suppressed. Moreover, vertical alignment is maintained even if a lotof ultraviolet light is irradiated from the front, and fluctuations inthe pretilt angles are small even if there are fluctuations in theamount of irradiated ultraviolet light.

A second embodiment of the present invention will now be explained withreference to FIGS. 10A and 10B.

As shown in FIG. 10A, the alignment of the liquid crystal layer 112 ofthe second embodiment is controlled to be in the same direction asalignment control by the electric field leaking from the gate electrodes113. In particular, the oblique electric field from the gate electrodeor data electrode assists in the orientation. Consequently, thealignments near the gate electrodes 115 change continuously anddisclinations do not develop. However, when the tilt of the liquidcrystal in FIG. 10A is reversed, disclinations develop near region 114a. See, FIG. 10B. In other words, if the orientations are controlled tooppose the orientations caused by the oblique electric field from thegate electrode or data electrode, disclinations will develop. Incontrast, according to in this embodiment, the alignment is controlledover the entire display electrode surface because photo-alignment isused. Consequently, response is fast in this embodiment, anddisclinations do not occur.

In this display shown in FIGS. 10A and 10B, the inclination of the sidesurface of rib-shaped part 116 is used to control the alignment of theliquid crystal. If only the rib-shaped part 116 is used, the gap with anadjacent rib-shaped part 116 must be narrower. For example, a gap around30 μm is preferred. In this case, however, the rib-shaped parts areoften present in the display pixels.

In this embodiment, the gaps between the rib-shaped parts can be widenedbecause they in combination with the photo-alignment. Duringphoto-alignment, the rib-shaped parts do not necessarily have to have anactive alignment control force. Alignment by photo-alignment providesthe possibility of not determining with certainty the position in thecenter part shown in FIG. 10A. For example, if the width of the slit inthe optical mask is about 20 μm, the center of the domain is believed tobe difficult to reliably bring to the center of the slit. The positionsof the divisions in this domain are reliably set by forming the rib andthe rib-shaped part plays a major role in this embodiment.

As described above, the alignment film for photo-alignment is preferablya vertically or horizontally aligned polyimide, polyamic acid, orcross-linked resin film (for example, polyvinyl cinnamate). As will beappreciated by one of ordinary skill in the art, the materials are notlimited to those listed above. Moreover, the alignment need not belimited to vertical and horizontal alignments.

According to a preferred embodiment, the structure includes verticallyaligned polyimide. Specifically, the alignment of the polyimide ispreferably vertical in an initial state. Moreover, the liquid crystalprovided between the alignment films preferably have negative dielectricanisotropy, particularly a fluorine liquid crystal In addition, thematerial of the rib-shaped part is a positive photoresist.

The liquid crystal panel structure shown in FIG. 10A includes arib-shaped part 116 in addition to the above-described alignment film.The rib-shaped part 116 assists in producing the two domains. As a basicstructure, this idea may also applied to producing four domains. Theliquid crystal molecules in the liquid crystal layer 114 are set to tiltfrom the upper and lower gate electrodes 113 of the pixel electrode 118towards the center of the pixel, or (and) the liquid crystal moleculesare set to tilt from the left and right data electrodes 115 of the pixelelectrode 118 towards the center of the pixel.

A slit 111 (not illustrated in FIG. 10A, see, FIG. 11) in the opticalmask 102 on the TFT substrate 104 b side is provided at the center ofthe pixel and ultraviolet light is irradiated. Similarly, ultravioletlight is irradiated at an incline on the opposing CF (color filter) sidesubstrate 104 a. The resin rib-shaped part 116 can be provided on theTFT substrate 104 b and/or the CF (color filter) substrate 104 a toassist in controlling the alignment direction.

FIGS. 11A and 11B depict the light source used in the alignment processof the present invention. FIG. 11A is a cross-sectional view along thelengthwise direction of the lamp, and FIG. 11B is a cross-sectional viewalong the widthwise direction of the lamp.

The lamp 121 shown in FIG. 11B is preferably low-pressure mercury lampmanufactured by Ushio Denki Co., Ltd., a shielding plate 122 is disposedbetween the tubular ultraviolet lamp 121 and the exposure target surfaceof the alignment film 103 to prevent the light from directly reachingthe exposure target. A so-called cold mirror 123 that does not reflectinfrared light is disposed at the back surface.

The lamp structure shown in FIG. 11A, irradiates ultraviolet lightperpendicular to the optical mask 102 in the widthwise direction of thelamp 121. The perpendicular irradiation directions to the ultravioletlight tube are nearly perpendicular to the substrate (FIG. 11A).However, the parallel irradiation directions to the ultraviolet lighttube are random (FIG. 11B). The slit 111 in the optical mask 2 and thelamp 121 are arranged orthogonal to each other. Light from the slit 111exposes the alignment film 103 at an incline in the form of leaks in thewidthwise direction of the slit 111.

As shown in FIG. 12, the lamp 121 is caused to scan the slit 111 (whilebeing maintained perpendicular to the slit 111) so as to uniformlyexpose the entire alignment film 3 with scattered light. The tube scansperpendicular to the slit openings in the mask. As will be appreciatedby one of ordinary skill in the art, other lamp structures may be used,without departing from the scope of the present invention. For example,the installation direction of the lamp 121 may be at 90°. Moreover, itis possible to remove the shielding plate 122 disposed directly belowthe lamp 121 to allow the ultraviolet light directed toward the surfaceof the alignment film 103 to be actively used while the scattered lightis directly irradiated from the lamp 121. However, by combining theshielding plate 122 and the cold mirror 123 of this embodiment, there isless possibility of light being irradiated in a different direction thanthe direction perpendicular to the desired direction for tilting theliquid crystal molecules, that is the elongated direction of the slit111. In addition, alignment is more stable and reliable.

When light enters from the slit 111, either polarized light ornon-polarized light is acceptable, but if the alignment film arranged inthe perpendicular direction is used, non-polarized light can be used.The light irradiation method is proximity exposure because the lightflows in and irradiates. The distance between the optical mask 102 andthe alignment film 103 is preferably several μm to 100 μm. If outside ofthis range, the inflow of light is inadequate, and negative effects suchas not obtaining the alignment and difficulty in specifying theboundaries of the domains may result.

The width of the slit 111 in the optical mask 102 is preferably severalμm to around 100 μm. If outside of this range, the incoming light issimilarly inadequate, and negative effects such as poor alignment anddifficulty in specifying the boundaries of the domains may result. Thedomains in this embodiment are described using examples applied to a TFTLCD.

FIGS. 13A, 13B and 13C show one example of a TFT LCD having two domainson the top and bottom alignment films. It should be appreciated that thelamp structure depicted in FIGS. 11A-12 is used to provide UVirradiation, and reference to slit 111 is understood to refer to theslit 111 in FIGS. 11A-12. FIG. 13A is an enlarged view near the pixelelectrode. FIG. 13B is a cross-sectional view when aligning on the CFsubstrate side. FIG. 13C is a cross-sectional view when aligning on theTFT substrate side.

In FIG. 13B, the slit 111 is provided parallel to and close to the gateelectrode 113 and irradiates ultraviolet light on the CF substrate 104a. In FIG. 13C, the slit 111 is provided at the position that coincideswith the Cs electrode 117 and is parallel to the storage capacitive (Cs)electrode 117 (gate electrode 113) and irradiates ultraviolet light onthe TFT substrate 104 b. In both FIGS. 13A and 13B, the scattered lightis irradiated so that liquid crystal molecules tilt in the directionfrom the gate electrodes 113 of the TFT substrate 104 b to the verticalcenter of the pixel electrode 118 of the CF substrate 104 a.

In addition, the rib-shaped part 116 may be provided and is effectivewhen installed parallel to the gate electrode 113 (Cs electrode 117)near the center of the pixel electrode 118 on the CF substrate 104 aside. Alternatively, the rib-shaped part 116 may be installed parallelto the gate electrode 113 (Cs electrode 117) at the position nearlycoinciding with the gate electrode 113 on the TFT substrate 104 b side.

FIGS. 14A-14C showing one example of two domains on the left and rightof a TFT LCD. It should be appreciated that the lamp structure depictedin FIGS. 11A-12 is used to provide UV irradiation, and reference to slit111 is understood to refer to the slit 111 in FIGS. 11A-12. FIG. 14A isan enlarged top view of the vicinity of a pixel electrode. FIG. 14B is across-sectional view during alignment on the CF substrate side. FIG. 14Cis a cross-sectional view during alignment on the TFT substrate side.

The slit 111 for ultraviolet light exposure on the CF substrate 104 aside is positioned parallel to the data electrode 115 and almostcoincides with the position of the data electrode 115 and transmitsscattered UV light. On the TFT substrate 104 b side, the slit 111 ispositioned parallel to the data electrode 115 at the horizontal centerof the pixel electrode 118 and transmits scattered UV light. Therefore,the liquid crystal molecules are oriented to tilt from the dataelectrodes 115 on the TFT substrate 104 b to the horizontal center ofthe pixel electrode 118 of the CF substrate 104 a. This coincides withthe alignment direction due to the oblique leaking electric field fromthe data electrodes 115. The alignment may further be stabilized byfixing the positions of the occurrences of disclinations at thealignment boundaries using the rib-shaped parts 116.

As best seen in FIG. 14C, the rib-shaped part 116 may be provided to runvertically along the center of the pixel electrode 118 on the CFsubstrate 104 a side, parallel to the data electrodes 115 at thepositions that nearly coincide with the data electrodes 115 on the TFTsubstrate 104 b side.

FIGS. 15A-16C are schematic drawings showing an example of four domainson the left, right, top, and bottom in a TFT LCD. FIG. 15A is anenlarged top view in the vicinity of a pixel electrode having therib-shaped element 116 fanned on the TFT side, and a rib-shapedprojection 155 formed on the CF substrate side. The solid arrow 152indicate the alignment direction of the TFT side, and dashed arrows 153indicate the direction of falling on the CS side. Moreover, arrows 154indicate the tilt direction when voltage is applied to the liquidcrystals.

FIG. 15B is a cross-sectional view during alignment along a dataelectrode.

FIG. 15C is a cross-sectional view during alignment along a gateelectrode. The state of the ultra violet light irradiating the TFTsubstrate is generally designated 156. It should be appreciated that thelamp structure depicted in FIGS. 11A-12 is used to provide UVirradiation, and reference to slit 111 is understood to refer to theslit 111 in FIGS. 11A-12.

If the CF substrate 104 a (FIG. 15B) is placed in the foreground of thepaper, the tilt for any of the liquid crystal molecules in FIGS. 15A-16C is in the alignment direction in which the liquid crystal moleculesfall from the four comers of the pixel electrode 118 of the TFTsubstrate 104 b towards the center of the pixel electrode 118. Fourdomains are produced on the top, bottom, left, and right sides of thepixel electrode 118. On average, the liquid crystal molecules in the topright region are aligned to fall from the northeast to the southwest.Similarly, the tilts of the liquid crystal molecules are aligned to fallfrom the southeast to the northwest in the lower right, from thesouthwest to the northeast in the lower left, and from the northwest tothe southeast in the upper left.

To make the liquid crystal molecules tilt at a 45° incline, theorientation on the CF substrate 104 a side and the orientation on theTFT substrate 104 b side have true directions at 90° so that the liquidcrystal molecules fall towards the center in these two directions. Theprinciple behind this alignment direction has been disclosed, forexample, in the Digest of AM-LCD98. If the liquid crystal molecules tiltfrom the northeast to the southwest, the two methods considered are (1)a method that aligns the TFT substrate 104 b side to fall towards thesouth and the CF substrate 104 a side to fall towards the west, and (2)a method that aligns the TFT substrate 104 b side to fall towards thewest and the CF substrate 104 a side to fall towards the south.

FIGS. 15A-15C show the alignment according to method (1). On the TFTsubstrate 104 b side, the slit 111 in the optical mask for ultravioletlight exposure is provided close to and parallel to the Cs electrode 117and irradiates scattered light (FIG. 15B). On the CF substrate 104 aside, the slit 111 in the optical mask for ultraviolet light exposure isprovided close to and parallel to the data electrode 115 (FIG. 15A) andirradiates scattered light.

As shown in FIG. 15C, it is effective to provide a rib-shaped part 116on both the TFT substrate 104 b and the CF substrate 104 a. On the TFTsubstrate 104 b side, the rib-shaped parts 116 are formed to be parallelto and close to the data electrode 115 and the gate electrode 113,respectively. Thus, they act to assist in creating four domains in theliquid crystal. On the CF substrate 104 a side, the rib-shaped parts 116are formed in a shape that extends vertically and horizontally from thecenter of the pixel electrode 118. As described earlier, the rib-shapedpart acts to promote establishing the boundaries of the domaindivisions.

FIGS. 16A-16C show alignment according to method (2), in which arib-shaped element 116 is formed on both the TFT substrate 104 b and CFsubstrate 104 a. The solid arrow 163 indicate the alignment direction ofthe TFT side, and dashed arrows 162 indicate the tilt direction of theliquid crystals on the CF side. Moreover, arrows 164 indicate the tiltdirection when voltage is applied to the liquid crystals.

On the CF substrate 104 a side, the slit 111 in the optical mask forultraviolet light exposure is provided close to and parallel to the gateelectrode 113 and irradiates scattered light (FIG. 16B). The state ofthe ultraviolet light irradiating the CF substrate side is generallydesignated 165.

On the TFT substrate 104 b side, the slit 111 in the optical mask forultraviolet light exposure is provided parallel to the data electrode115 near the horizontal center of the pixel electrode 118 (FIGS. 16A and16B). The rib-shaped parts 116 are formed close to and parallel to thedata electrode 115 and the gate electrode 113, respectively. Notably,the rib-shaped parts 116 assist in creating four domains in the liquidcrystal. On the CF substrate 104 a side, the rib-shaped parts 116 areformed in a shape that extends vertically and horizontally from thecenter of the pixel electrode 118. The state of the ultraviolet lightirradiating the CF substrate side is generally designated 165 (FIG.16C).

The surface energy of the alignment film in the pixel of the liquidcrystal display device having domains reaches a maximum at a domainboundary and reaches a minimum at a position separated from theboundary. The reason is the ultraviolet light exposure differs in apixel. The domain boundary is directly below the slit 111, and thesurface energy reaches a maximum because most of the ultraviolet lightirradiates this part. Because only leakage light irradiates the partseparated from the boundary, the absolute amount of ultraviolet lightexposure becomes smaller and the surface energy does not increase.

FIGS. 17A-17C shows a structure that suppresses disorder in thealignment caused by the lateral electric fields from the data electrodesusing the rib-shaped part 116 installed on the CF substrate side whenthere are two domains, an upper and a lower domain, in a TFT LCD. Thesolid arrow 171 indicates the alignment control direction, and arrow 162indicates the tilt direction of the liquid crystals.

FIG. 17A is an enlarged top view in the vicinity of a pixel electrode.FIG. 17B is a cross-sectional view during alignment along the dataelectrode (along line segment 17B-17B). FIG. 17C is a cross-sectionalview during alignment along the gate electrode (along line segment17C-17C in FIG. 17A).

As shown in FIG. 17A, on the CF substrate 104 a side, the rib-shapedparts 116 are formed horizontally in the center of the pixel electrode118 and parallel to the data electrodes 115 in the parts opposite thedata electrodes 115. The effect of the rib-shaped part 116 installedparallel to this data electrode 115 is explained with reference to FIG.17C. The liquid crystal molecules near the data electrode 115 have atendency to tilt towards the center of the pixel due to the electricfield from the data electrode 115. As shown by dashed portion 174 inFIG. 17C, the rib-shaped part 116 installed on the opposing CF substrate104 a tilt the liquid crystal molecules in the direction away from thepixel electrode 118. These effects cancel each other, and the liquidcrystal molecules do not fall towards the center of the pixel but tiltuniformly in the vertical direction.

As shown in FIG. 18, ends 116 a of a rib-shaped part 116 opposite a gateelectrode 115 are formed to be opposite to and partially overlap theends of the pixel electrodes 118 of the TFT substrate 104 b. FIG. 19shows the relationship between the width of this overlapping part 116 aand the width of a poor alignment at the end of a pixel electrode 118.In this case, by making the width of the overlapping part 116 a at least1 μm and more preferably 2 μm, the development of poor alignments can besuppressed. When the overlapping part 116 a is actually formed, themismatch is maintained around 3 μm and the width of the overlapping part116 a of at least 1 μm is reliably obtained. If the upper limit of theoverlapping part 116 a is set to 5 μm in order to not harm the functionof the pixel electrode, the width is designed to be 1 μm (lower limit ofthe required width)+3 μm (mismatch) to 5 μm (upper limit of the requiredwidth)+3 μm (mismatch)=from at least 4 μm to no more than 8 μm or fromat least 5 μm to no more than 8 μm. Therefore, the generation of pooralignments can be adequately prevented.

The structure forming the rib-shaped part 116 on the CF substrate 104 aor the TFT substrate 104 b has been described. However, similar resultscan be obtained without the rib-shaped parts 116 by removing slit shapesthat are not part of the electrode in the pixel electrode 118. Notably,FIG. 20A corresponds to FIG. 13A when the slit-shaped notch 131 wasformed parallel to the Cs electrode 117 (gate electrode 113) of thepixel electrode 118 and at the position coinciding with the Cs electrode117. FIG. 20B corresponds to FIG. 14A when the slit-shaped notch 131 wasformed in the pixel electrode 118 parallel to the data electrode 115 andat the position corresponding to the center of the pixel electrode 118.FIG. 20C corresponds to FIG. 15A when the slit-shaped notch 131 shapedas a cross was formed in the pixel electrode 118.

FIG. 21 is a characteristic plot showing the test results about thewidth of the slit 111 in the optical mask with an excellent alignmentstate and the optimum value of the distance (distance A) between theoptical mask and the substrate. Excellent alignment can be obtained fora slit width from 3 μm to 100 μm (Region A) and a distance between themask and the substrate from 3 μm to 100 μm. Furthermore, the preferreddistance between the optical mask and the substrate is from 50 μm to 100μm (Region 21B). The slit width and the distance A are nearly equal.Excellent alignment can be obtained when the slit width is set in therange from about the same value as distance A to about 1/20^(th) ofdistance A.

According to this embodiment as described above, alignment usingultraviolet light is accurately performed in the minimum number ofprocesses. A vertically aligned liquid crystal display device isimplemented few disclinations in two or four domains. The result is theability to produce a superior bright screen when using the TN mode.Furthermore, the response speed can provide high-speed responsivenessthat is similar to or better than in a so-called MVA liquid crystaldisplay device provided with many rib-shaped parts.

FIG. 22 is a cross-sectional view of the main structures in thealignment apparatus of another embodiment.

In this embodiment, the alignment film is a copolymer of two polymers.Preferably, the alignment of one polymer changes from the initialvertical alignment in response to ultraviolet light exposure and assumesa constant value of approximately 90° when the ultraviolet lightexposure exceeds some level. The other polymer has the property ofstarting to change the pretilt angles from the vertical alignment inresponse to ultraviolet light exposure, and returning again to thevertical alignment when exposed again to ultraviolet light.

Quartz glass which is transparent in the short wavelength region (forexample, 254 nm) of the ultraviolet light is preferably used for theoptical mask 201. A mask pattern of metallic chromium is formed on oneside of the optical mask 201. The mask pattern provides a stripe-shapedslit 211 in the metallic chromium. The stripe-shaped slits 211 are linedup at the same pitch as the pitch of the pixels having domains. As oneexample, if the pixel pitch is 200 μm, the width of a slit 211 is 10 μm,and the width of the metallic chromium pattern becomes 190 μm from oneslit 211 to the adjacent slit 211.

The scattering mechanism 221 that scatters parallel light is formed onthe surface of the optical mask 201 on the light source side. Scatteringof the incident light is generally shown by the dashed circle 22 a. Thescattering mechanism 221 may be formed, for example, by sandblasting thesurface of the optical mask 201 on the light source side.

The alignment film 203 is irradiated with ultraviolet light on the glasssubstrate 202. The optical mask 201 is placed so that the position ofthe stripe-shaped slot 211 almost coincides with the horizontal centerposition of the pixel and parallel to the data electrode.

The optical mask 201 is placed in the direction parallel to the dataelectrode with the position of the stripe-shaped slit 211 at theposition of the data electrode of the TFT substrate 204B in the opposingsubstrate 204A when ultraviolet light irradiates the opposing substrate(CF substrate) 204A side.

After the optical mask 201 is disposed as described above, parallelultraviolet light is irradiated perpendicular to the surface of theoptical mask 201 on the light source side. The irradiated ultravioletlight is scattered by the ground glass part and is split into twodirections with the center as the boundary from the slit 211 andirradiated as illustrated.

When the TFT substrate 204B is affixed to the opposing substrate 204A,the positions of corresponding slits become the centers of the pitchlining up the slits. Therefore, the region inclined in the directionperpendicular to the slit can be between the slit on the TFT substrate204B side and the slit on the opposite substrate 204A side, that iswithin a 90 μm width. The domains in two orientations can be produced inone pixel by giving the orientations mutually opposite directions withthe position of the slit in the center of the pixel as the boundary.

FIG. 23 is a cross-sectional view showing another example of thisembodiment. The optical mask 201 is disposed as described above andirradiates parallel ultraviolet light perpendicular to the surface ofthe optical mask 201 on the light source side. Irradiated ultravioletlight is scattered in two directions by the slit 211, and exposes thealignment film 203.

If the TFT substrate 204B is affixed to the opposing substrate 204A, thepositions of their mutual slits are at the centers of the pitch liningup the slits. Thus, the region inclined in the direction perpendicularto the slit can be between the slit on the TFT substrate 204B side andthe slit on the opposing substrate 204A side, that is within a 90 μmwidth. Domains in two directions can be produced in one pixel by givingthe orientations mutually opposite directions with the position of theslit in the center of the pixel as the boundary.

FIG. 24 is a cross-sectional view showing another example of thisembodiment.

A prism 212 generally shaped like an isosceles triangle with the widthof the slit opening as its base is disposed in the opening of the slit211 in the optical mask 201 described above.

Similar to the above description, the position of the optical mask 201is perpendicular to the surface of the optical mask 201 on the lightsource side and irradiates parallel ultraviolet light. The irradiatedultraviolet light is reflected and refracted by the prism 212 and issplit into parallel light in two directions when the scattered light isemitted from the prism 212 as shown in the drawing and exposes thealignment film 203.

If the TFT substrate 204B is affixed to the opposing substrate 204A, thepositions of their mutual slits are at the centers of the pitch liningup the slits. Therefore, the region inclined in the directionperpendicular to the slit can be between the slit on the TFT substrate204B side and the slit on the opposing substrate 204A side, that iswithin a 90 μm width. Domains in two directions can be produced in onepixel by producing orientations having mutually opposite directions withthe position of the slit in the center of the pixel as the boundary.

Even if the ultraviolet light that exposes the surface of the opticalmask 201 on the light source side is scattered light in this embodiment,similar to when parallel light is irradiated, the ultraviolet lightexposing the alignment film 203 can be split in two directions toproduce the desired domains. This method disperses the ultraviolet lightthat exposes the alignment film 203 directly below the slit, and theultraviolet light exposure on this part no longer becomes excessive. Inaddition, domains can be produced by one exposure on one substrate.

As described above, even if the ultraviolet light on the optical mask201 is parallel light, similar effects can be obtained when scatteredultraviolet light irradiates the optical mask 201 by scattering, orreflection or refraction by the ground glass part of the optical mask201 or the prism 212. This shows that an ultraviolet light exposuredevice that emits parallel light as the light source can be used.

Because the ultraviolet light can be dispersed in the part of thealignment film 203 in the opening of the slit 211, excess exposure lightin this part can be prevented. And void areas caused by a lower tilt inthis part and flow-induced orientations can be prevented.

Embodiment 4

Yet another embodiment will be explained with reference to FIGS. 25A-26.Similar to the second embodiment, the alignment film in this embodimentis composed of two polymers. Preferably, alignment of one polymerchanges from an initial vertical alignment in response to ultravioletlight exposure, and assumes a constant value near 90° when apredetermined level of ultraviolet light exposure is exceeded. Thealignment of the other polymer starts to change from the verticalalignment in response to ultraviolet light exposure, but returns to thevertical alignment when exposed again to ultraviolet light.

FIG. 25A is a cross-sectional view of the main structures in thealignment apparatus in this embodiment. FIG. 25B is a cross-sectionalview of the main structures in the liquid crystal display deviceimplementing this alignment method. In FIGS. 25A and 25B, the liquidcrystal molecule is generally designated 251.

Quartz glass that has the property of transmitting the short wavelengthregion (for example, 254 nm) of ultraviolet light is the material of theoptical mask 301. As shown in FIG. 26, a mask pattern made of metallicchromium is formed on one surface of the optical mask 301. The maskpattern is provided with a stripe-shaped alignment control slit 211 inthe metallic chromium. The alignment control slit 211 orients the liquidcrystal molecules in the desired direction, and is lined up at the samepitch as that of the pixels having domains. As an example, if the pixelpitch is 200 μm, then the width of the slit 211 is 10 μm and the widthof the metallic chromium mask pattern from a slit to an adjacent slit211 becomes 190 μm.

Furthermore, an alignment correction slit 311 is provided in the sameoptical mask 311. This slit must be finer than the slit that orients theliquid crystal molecules in the desired direction and must be disposedin a mutually perpendicular direction. The pixel pitch is aboutone-third at 70 μm, and the width of the slit 311 is about 1 μm.

Next, ultraviolet light irradiates the alignment film 303 on the glasssubstrate 304. As shown in FIG. 27, the optical mask 301 is placed sothat the position of the alignment control slit 211 almost coincideswith the center position horizontally of the pixel and is perpendicularto the data electrode 315 when ultraviolet light irradiates the TFTsubstrate 304 b side. The positions of the alignment correction slits311 are parallel to the data electrodes 315 and centered betweenadjacent pixel electrodes 318, that is they are at the centers of thedata electrodes 315.

The optical mask 301 only needs the alignment control slit 211 whenultraviolet light irradiates the opposite substrate 304 a side. Theposition of this slit 211 is the position of the gate electrode 313 ofthe TFT substrate 304 b in the opposite substrate 304 a and is disposedin the direction perpendicular to the data electrode 315.

After the optical mask 301 is disposed as described above, scatteredultraviolet light is irradiated perpendicular to the surface of theoptical mask 301 on the light source side. As shown in FIG. 25A, theirradiated ultraviolet light is split into two directions by the slit311.

The ultraviolet light irradiates through the slit 311, and spreads outin a fan shape centered on the slit 311. As shown in FIG. 25, thealignment control force that tilts the liquid crystal molecules in somedirection of the slit 311 is applied to the alignment film 303. Thus,the directions of the force that orients the liquid crystal molecules bythe electric field at the ends of the pixel electrode 318 and thealignment control force of the alignment film 303 oppose each other inorder to cancel the forces that orient the liquid crystals, tilting theliquid crystal molecules perpendicular to the direction of the desiredtilts of the liquid crystal molecules can be prevented.

The brightness changes in the image display device (device A) thatimplements the alignment connection in addition to the alignment controlof two divisions by the method of this embodiment is examined based on acomparison with the image display device (device B) that implements onlyalignment control for two divisions. In device B (FIG. 28A), the tilt ofthe liquid crystal molecules by the electric fields at the ends of thepixel electrodes 318 is generally designated 281. The desiredorientation of the liquid crystal molecules is generally designated 282.Moreover, regions where brightness reductions occur at the ends of theblack matrix 321 are generally designated 281 in FIG. 28B.

In contrast, the tilts of the liquid crystal molecules are eliminated atthe ends of the pixel electrodes 318 in FIG. 29A (Device A).Consequently, an excellent image is obtained without the problem oflower brightness regions at the ends of the black matrix 321.

If the TFT substrate 304 b is affixed to the opposing substrate 304 a,the positions of their mutual slits are at the centers of the pitchlining up the slits. The region that inclines in the directionperpendicular to the slit can be between the slit on the TFT substrate304 b side and the slit on the opposing substrate 304 a side, that iswithin a 90 μm width. Domains in two directions can be produced in onepixel by producing orientations having mutually opposite directions withthe position of the slit in the center of the pixel electrode 318 as theboundary.

FIG. 30 is a top view of the optical mask in another example of thisembodiment. Although similar to the above description up to forming theslit in the optical mask, the scattering mechanism for the incidentultraviolet light is provided on the surface on the side opposite thealignment film 303 of the slit 211 that orients the liquid crystalmolecules in the desired direction. As a specific example of thescattering mechanism, a groove with a depression-shaped cross-section isprovided by sandblasting only the opening of the slit 211, forming theground glass part 211 a, or irradiating with laser pulses.

The irradiated ultraviolet light is scattered by the ground glass part211 a and has its irradiation width narrowed by the alignment correctionslit 211, so it does not negatively affect the essential orientations ofthe liquid crystal molecules.

FIG. 31 is a top view of the optical mask in another example of thisembodiment.

Only the alignment correction slit 311 in the optical mask 301 has aprescribed height, for example, it is formed to be about 50 μm.Therefore, the gap between the optical mask 301 and alignment film 304,which are opposite each other, becomes narrower to around 50 μm only inthe alignment correction slit 311 and can narrow the width of thescattered ultraviolet light incident on only this part. Thus, theessential orientations of the liquid crystal molecules are notnegatively affected.

FIGS. 32A and 32B are schematic diagrams of the light source accordingto another used embodiment. FIG. 33 is a top view showing therelationship between the scattering of the light source and the slit inthe optical mask.

A method for changing the direction of the tubular light source 302 isadopted. The lengthwise direction of the light source 302 (FIG. 32B) hashigher scattering than the widthwise direction of the light source 302(FIG. 32A). This characteristic is used in this embodiment.

The lengthwise direction of the light source 302 for ultraviolet lightis positioned to be parallel to the direction of the alignmentcorrection slit 311 in the optical mask 301. Thus, the ultraviolet lightpassing through the alignment correction slit 311 has a narrowscattering width and is positioned perpendicular to the lengthwisedirection of the light source 302. The ultraviolet light that passedthrough the slit 211 that orients the liquid crystal molecules in thedesired orientations has a wider scattering width. Therefore, there areno negative effects on the essential orientations of the liquid crystalmolecules.

According to this embodiment as described above, because the directionsof the alignment control force at the end of the pixel electrode 318 andthe force to orient due to the electric field cancel each other, tiltingthe liquid crystal molecules in the direction perpendicular to thedesired tilt direction for the liquid crystal molecules can beprevented. Therefore, the occurrence of disclinations is prevented, anda decrease in the brightness at the ends of the pixels can besuppressed.

Because new rib-shaped parts do not have to be formed, the process thatcontrols the alignment can be simplified by forming the alignmentcorrection slit 211 and the alignment control slit 311 in the opticalmask 301.

A further embodiment of a liquid crystal display device featuring apixel electrode is described with reference to FIG. 34.

The alignment film in this embodiment is composed of two polymers.Preferably, the pretilt angle of one polymer changes from an initialvertical alignment in response to ultraviolet light exposure and assumesa constant value near 90° when some level of ultraviolet light exposureis exceeded. The pretilt angle of the other polymer starts changing fromthe vertical alignment in response to ultraviolet light exposure, butresumes vertical alignment when exposed again to ultraviolet light.

To prevent poor orientation caused by the lateral electric field fromthe data electrode 415, a slit 411 is disposed near the data electrodes415 of the pixel electrode 418. The slit 411 extends in the directionparallel to the data electrode 415 (direction orthogonal to the gateelectrode 413). An effective width for this slit 411 falls in the rangefrom 2 μm to 5 μm. In particular, a 3 μm width for the slit wasconfirmed as the most effective size for suppressing poor orientation.

The provision of a fine slit 411 in the pixel electrodes 418 has theproperty of making the liquid crystal molecules fall in the directionparallel to this slit 411. In this embodiment, this action is jointlyused with the photo-alignment. Essentially, if there is liquid crystalin the gaps between the pixel electrodes, the liquid crystal tilts inthe direction away from the gap because the electric field becomesoblique (see FIG. 35A). However, if a fine slit, for example, a 3 μmwide slit 411, is provided, the liquid crystal molecules will tilt onboth sides of a slit 411 and the destinations disappear. Consequently,the liquid crystal molecules are oriented to tilt towards the slitdirection (see FIG. 35B). If a slit 411 is provided as shown in FIG. 34,the tilts of the liquid crystal molecules similarly lose theirdestinations because of this slit (FIG. 35B). Consequently, the liquidcrystal molecules will tilt in the direction parallel to the slit 411,and poor orientation caused by the data electrode is suppressed.

FIG. 36 is a schematic showing another example of this embodiment. FIG.36A is a top view of the vicinity of a pixel electrode. FIG. 36B is across-sectional view.

A plurality of slits 411 is provided over the entire pixel electrode 418area. Therefore, the stability of the orientation becomes more reliable.It is important for these slits 411 to be connected by the connector 421in the center of the pixel electrode 418. If the relationship betweenthe connector 421 and the slits 411 is examined, the electric field atthe connector 421 is as shown in FIG. 34B and expands in a fan shapefrom the connector 421. This effect tilts the liquid crystal moleculesin a more preferred direction.

According to this embodiment as described above, a liquid crystaldisplay device with no poor orientations and a wide viewing angle can beimplemented.

While various embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

Various features of the invention are set forth in the appended claims.

1. A liquid crystal display device comprising: a pair of substrates in apredetermined spaced relationship with each other; a pair of alignmentfilms, one said alignment film formed on each said substrate such thatsaid alignment films face each other; and a liquid crystal layerinserted between said pair of alignment films, said alignment filmsbeing comprised of a material containing at least two types of polymershaving different variation rates of a pre-tilt angle in response toultraviolet light irradiation, wherein one type of said two types ofpolymers vertically orients liquid crystal molecules of said liquidcrystal layer in an initial state, and the other type of said two typesof polymers horizontally orients the liquid crystal molecules of saidliquid crystal layer in the initial state.
 2. The liquid crystal displaydevice according to claim 1, wherein said polymer vertically orientingthe liquid crystal molecules of said liquid crystal layer changes theorientation of the liquid crystal molecules from the initial state whenirradiated with the ultraviolet ray, and said polymer horizontallyorienting the liquid crystal molecules of said liquid crystal layerhardly changes the pre-tilt angle when irradiated with the ultravioletray.
 3. The liquid crystal display device according to claim 1, whereinsaid polymer vertically orienting the liquid crystal molecules of saidliquid crystal layer has an alkyl group in a side chain thereof.
 4. Theliquid crystal display device according to claim 1, wherein saidalignment films are comprised of a material containing a polymermixture.